Carrier aggregation (CA) is an important feature for bandwidth expansion and throughput increase in wireless communications technologies such as those governed by 3GPP (3rd Generation Partnership Program) LTE (Long Term Evolution) and LTE-advanced standards. CA has been developed to meet growing capacity demands due to rapid growth of wireless data services.
In CA, more than one carrier is employed simultaneously to carry information between CA capable wireless devices and radio access nodes. Wireless devices are also referred herein as user equipment devices (UEs) while radio access nodes may be referred herein as base stations, evolved or enhanced Node B's (eNB's), remote radio units (RRUs) etc. CA introduces the concepts of Primary cell (PCell) and Secondary cells (SCells). The PCell and SCell are UE specific. The PCell for a UE is the cell that the UE selects to camp on. By design, a PCell stays activated all the time, handling the radio resource control (RRC) connection establishment, re-establishment, or handover. If a UE is CA capable, one or more SCells can be allocated to a session for the UE, such as via RRC messages. The SCell of a UE can be in an activated or a deactivated state. In LTE, SCell activation/deactivation commands are sent to a UE via the medium access control (MAC) layer. An SCell can carry data only after the SCell activation.
A carrier-aggregation capable LTE UE needs to monitor the control channels (such as Physical Downlink Control Channels (PDCCH)) and the Common Reference Signal (CRS) for activated SCells, in addition to monitoring a control channel for the PCell, in each downlink sub-frame for possible DL data transmissions. These monitoring activities consume the UE's battery power. Therefore, from a UE power saving perspective, it is important that an SCell for the UE is deactivated as soon as new data request for the UE is low or whenever new data request can be handled fully by the PCell.
Apart from considering the UE new data request, a radio access node also needs to consider how to perform link adaptation (LA) to more effectively use radio resources. One strategy is to deactivate a UE's SCell when the UE is in a bad radio frequency (RF) condition. The radio access node may determine a UE's RF condition by a combination of UE's channel condition reports, such as channel quality information (CQI) or channel state information (CSI) reports, and an outer-loop adjustment value from an outer-loop link adaptation (OLA) algorithm. Often, the outer-loop adjustment value is a signal-to-noise ratio (SNR) correction factor (SNR_OLA), proposed as an adjustment factor for a better SNR estimation, as channel condition reports such as CQI reports might not be accurate and, for example, might not consider inter-cell interference. Herein, SNR can also represent signal-to-noise-and-interference-ratio (SINR). In LTE downlink (DL) CA for example, for a given UE, link adaptation can be based on a signal-to-noise ratio (SNR) estimate which is the sum of an SNR mapped from downlink CQI and the outer-loop adjustment value. The CQI is determined by the UE and is reported periodically or aperiodically to the radio access node through uplink (UL) channels, such as Physical Uplink Shared Channel (PUSCH) or Physical Uplink Control Channel (PUCCH). A link adaptation processor receives downlink data for a downlink transmission to the mobile terminal scheduled for a particular sub-frame, selects a modulation and coding scheme (MCS) for the downlink transmission based on the SNR estimate, and outputs the downlink transmission with the selected MCS. When the SNR estimate calculated by the radio access node for a UE is very low, the link adaptation will schedule very small data rate for the UE. In this case, it may be better not to schedule SCell data to the UE in order to free up SCell data resources for other UEs. Furthermore, deactivating the SCell in such circumstances would also save the UE's battery power.
FIG. 1 illustrates a cellular communications network 1 representing a current LTE advanced CA scenario. The cellular communications network 1 includes a PCell 3 and at least one SCell 4. There may be more than one SCell, although FIG. 1 shows one as an example. The PCell 3 has associated a PCell coverage area and a PCell radio unit (RU) 5. The SCell 4 has associated an SCell coverage area and an SCell RU 6. RU's 5 and 6 are comprised within one or more radio access nodes (RANs) 7. The PCell and SCell coverage areas are distinct in terms of associated frequency carriers and may or may not be distinct in terms of physical geographical area. The RAN(s) 7 may comprise hardware and/or software components that are physically collocated or distributed at different physical locations. Furthermore, in the case of a plurality of RANs 7, the RANs may be physically collocated or distributed. A UE 2 can receive downlink (DL) signals from both the PCell 3 and the SCell 4. The UE 2 sends uplink (UL) signals only to the PCell 3. In the illustrated scenario, the signals sent back to the PCell from the UE 2 include at least one of:                Downlink (DL) CQI or CSI for PCell 3 and SCell 4; and        HARQ feedbacks for PCell 3 and SCell 4 (to indicate whether or not at least a transport block, i.e., a data portion, is successfully received and decoded).The feedback information received at PCell 3 for SCell 4 will be passed to SCell 4. A HARQ feedback may be, for example, either a positive HARQ acknowledgement (i.e., a HARQ ACK) or a negative HARQ acknowledgement (i.e., a HARQ NACK).        
OLA algorithms are done independently for each UE for PCell 3 and SCell 4. An OLA algorithm can be based, for example, on the HARQ feedback. A HARQ ACK may indicate that the current OLA value needs to be incremented, while a HARQ NACK may indicate that a current OLA value needs to be decremented. The OLA results for a UE are used in the link adaptation along with UE's CQI to determine MCS for maintaining best throughputs based on the radio channel conditions.”
In summary, existent schemes for SCell activation/deactivation known to the inventors, consider the following:                New data requests for the UE.        Downlink SNR for the corresponding SCell, based on UE reported CQI (SNR_CQI) and an outer-loop SNR adjustment/offset (SNR_OLA).        
In certain scenarios, however, the existent solutions for SCell activation/deactivation have some problems. Some exemplary scenarios are when UEs are moving or when UEs are travelling through high interference zones. In particular, in such scenarios, it has been observed that SCells of UEs are sometimes not able to re-activate and their throughput becomes zero or that SCells of UEs do re-activate, but high decoding errors are encountered at the beginning of the re-activation.
Therefore, there is a need in the art for improved methods and apparatus for SCell activation/deactivation.